The present invention relates generally to the field of conveyor devices, and more particularly, but not by way of limitation, to a roller and belt conveyor system for assembling printed circuit boards for a disc drive.
Modern hard disc drives are commonly used in a multitude of computer environments ranging from super computers through notebook computers to store large amounts of data in a form that can be made readily available to a user. Typically, a disc drive comprises one or more magnetic discs that are rotated by a spindle motor at a constant high speed. The surface of each disc serves as a data-recording surface and is divided into a series of generally concentric recording tracks radially spaced across a band between an inner diameter and an outer diameter. The data tracks extend around the disc and data is stored within the tracks on the disc surface in the form of magnetic flux transitions. The flux transitions are induced by an array of transducers otherwise commonly called read/write heads. Typically, each data track is divided into a number of data sectors that store fixed sized data blocks.
Each read/write head includes an interactive element such as a magnetic transducer, which senses the magnetic transitions on a selected data track to read the data stored on the track. Alternatively, the read/write head transmits an electrical signal that induces magnetic transitions on the selected data track to write data to the track. As is known in the art, the read/write heads are supported by rotary actuator arms and are positioned by the actuator arms over a selected data track to either read or write data. The read/write head includes a slider assembly having an air-bearing surface that causes the read/write head to fly relative to the disc surface. The air bearing is developed by load forces applied to the read/write head by a load arm interacting with air currents produced by disc rotation.
Typically, several open-centered discs and spacer rings are alternately stacked on the hub of a spindle motor, followed by the attachment of a clampring to form a disc pack. The hub, defining the core of the stack, serves to align the discs and spacer rings around a common centerline. Movement of the discs and spacer rings is typically constrained by a compressive load maintained by the clampring. The complementary actuator arms of an actuator assembly, commonly called an E-block, support the read/write heads to access the surfaces of the stacked discs of the disc pack. The read/write heads communicate electronically with a printed circuit board assembly (PCB) through read/write wires and a flex circuit attached to the E-block.
The PCB functions as an electronic interface between the magnetically stored data on the disc drive in the computer served by the disc drive. Additionally, the PCB provides electronic controls, such as read/write channels and servo circuits, for the physical operation and control of the disc drive during operation of the disc drive within the computer. The PCB monitors and controls the rotational speed of the spindle motor, the acceleration, deceleration and track following of the read/write heads, otherwise known as head positioning, by controlling the movement of the E-block. The speed, precision, reliability and data rate demanded of a modern disc drives for storage and retrieval of digital data has led to the utilization of increasingly sophisticated electronic components.
Reductions in form factor of modern disc drives has placed a premium on the availability of physical space, sometimes referred to as board real estate or real estate, for mounting components on PCBs. The scarcity of component mounting real estate has led to the incorporation of surface mount technology, as the preferred production technique for assembling PCBs used in disc drives. Surface mount technology accommodates the use of both sides of the circuit board for mounting electronic components. Additionally, surface mount technology is particularly suited for automated assembly of disc drives PCBs for two reasons. First, the physical size of surface mount components is characteristically small, making manual placement of the components on the circuit board difficult and time consuming and expensive. Second, the level of sophistication and susceptibility to mechanical and electrostatic damage of surface mount components necessitates the use of automation for the production of reliable PCBs. Collectively, the physical size of surface mount electronic components, the degree of precision required for placement of the surface mount electronic components, the production rates and the susceptibility to damage precludes the use of manual labor in the production process.
The demand for disc drives in the marketplace has grown faster than the demand for computers due to the increased presence, use and demand for Internet access. As a result, disc drive manufacturers have experienced a rapid increase in the demand for their products and are continually seeking ways to improve product throughput rates and productivity. One technique used by the industry to increase the throughput rate of PCBs is to process multiple printed wiring board as a single printed wiring assembly and separate the completed assembly into PCBs upon completion off the assembly process. A common configuration for printed wiring assemblies used by disc drives is a single printed wiring assembly made up of two printed wiring boards, also referred to as a xe2x80x9ctwo-upxe2x80x9d, with each printed wiring board including a combination interface and power board edge connector. Other typical configurations of printed wiring boards for printed wiring assemblies include xe2x80x9cfour-upsxe2x80x9d, xe2x80x9csix-upsxe2x80x9d and xe2x80x9ceight-upsxe2x80x9d. The number of printed wiring boards included in a printed wiring assembly depends on the physical size of the printed wiring board in relation to the overall dimensions of the printed wiring assembly the production line is capable of processing. For example, the stroke of the solder past application equipment often restricts the overall length of the printed wiring assembly and the conveyor width often restricts the overall width of the printed wiring assembly.
Another response by disc drive producers to the increased demand for disc drives has been the advances made in flexible, surface mount assembly lines. The concept behind a flexible, surface mount assembly line is the ability to intermix and reposition several types of automation equipment throughout the automated assembly line. The ability to xe2x80x9cmix and matchxe2x80x9d automation equipment to meet the product requirements of a particular print circuit board has allowed the manufacturers to shift between various PCB configurations while minimizing line setup times. However, at times, difficulties arise when coupling various types of equipment together in the production line and, in some instances, a dwell time is needed between process steps; or a time variable process necessitates the use of specialty conveyors specifically suited for surface mount technology automated assembly production line environments.
The following are among the requirements for surface mount technology conveyors; the conveyors should convey the PCBs with a minimum amount of jostling; the conveyors should be ESD safe; and should be designed to avoid jamming, scuffing, and shingling during transport and accumulation of PCBs thereon. An example of a typical conveyor meeting these basic needs is the Slip-Torque conveyor by Shuttleworth. Construction of the Slip-Torque conveyor includes a series of parallel rotating roller shafts with each roller shaft containing several adjacent loose fit rollers. The rotating roller shafts rotate under power and the coefficient of friction between each rotating roller shaft and loose fitting rollers controls the amount of drive force transmitted to the PCBs. The product weight and roller section determines the level of drive force.
Typically between 1 and 20 percent of the weight of the PCB applied to the loose fit rollers normal to the direction of travel is sufficient to actively convey the PCB. When an opposing force equal to between 5 to 25 percent of the weight of the PCB is applied to the PCB, opposite to and parallel with the direction of travel of the PCB, the weight of the board is no longer sufficient to sustain conveyance. The loose fit rollers cease rotation, the rotating roller shafts continued to rotate internal to the loose fit rollers and the PCB remains undisturbed until the opposing force ceases to be applied.
However, instances occur during the production of PCBs when a conveyor is needed; yet conventional conveyors, including conveyors constructed with loose fit rollers mounted on rotating roller shafts, fail to provide a solution. Typical belt conveyors are inappropriate for use as an accumulating conveyor and conventional power roller conveyors are problematic from a jamming, scuffing, and shingling perspective. While the loose fit rollers/rotating roller shaft conveyors address the scuffing and shingling issues the jamming issue remains unresolved.
Jamming often occurs when the geometry of the PCBs being assembled have a board edge connector attached on a leading or trailing edge. Board edge connectors often protrude below the horizontal surface of the PCB. When a board edge connector enters a valley between two adjacent parallel roller assemblies, there can be insufficient surface area between the PCB wiring board and the conveyor. Frictional force between conventional loose fit rollers and the rotating roller shafts is insufficient to convey the PCB, resulting in jamming. Freeing production line jams requires operator intervention, exposing the PCB to both physical and electrostatic damage and decreased productivity of the production line. There exists an unmet need for an accumulating conveyor capable of transporting surface mount technology PCBs that have components protruding below the PCB, and particularly for transporting PCBs with board edge connectors attached to the leading and trailing edges.
The present invention provides a roller and belt conveyor system well suited for conveying irregularities such as printed circuit board assemblies. The roller and belt conveyor system has a power drive assembly, supported by a frame, delivering rotational power to a number of roller assemblies that have several conveyor belt sections, each conveyor belt section supported by at least two roller assemblies. The conveyor belt sections convey the items between adjacent roller assemblies and over the length of the roller and belt conveyor system.
In a preferred embodiment, each roller assembly includes a powered rotating shaft that supports the number of adjacent loose fit rollers that have inner diameters greater than the outer diameter of the rotating shaft for slippage between the loose fit rollers and the power to rotating shaft. Each conveyor belt section circumvents at least one loose fit roller of each roller assembly supporting the conveyor belt section and each conveyor belt section is supported by at least one roller assembly that differs from the roller assemblies supporting adjacent conveyor belt sections. The coefficient of friction between the rotating shaft and each loose fit roller controls the amount of driving force transmitted from the power drive assembly to the loose fit rollers while the weight of the article determines the driving force available for conveying the article. The conveyor belt section inner circumference is 2xcex1(x+xcfx80r) where x is the distance between the rotational axes of the first and last roller assemblies supporting the conveyor belt section, r is an outer diameter radius of the loose fit roller and xcex1 is between about 1.00-1.05. Other features and advantages of this and other embodiments will become apparent upon a review of the attached drawings and accompanying description.